Analysing the assembly of cenocrons in the Mexican transition zone through a time-sliced cladistic biogeographic analysis

Abstract

A transition zone shows the overlap between two or more regions and represents an event of biotic hybridisation, where different cenocrons assembled as a result of historical and ecological processes. The Mexican transition zone, the area where the Nearctic and Neotropical regions overlap, includes the following five biogeographical provinces: Sierra Madre Occidental, Sierra Madre Oriental, Sierra Madre del Sur, Transmexican Volcanic Belt and Chiapas Highlands. Within this transition zone, the following five cenocrons have already been recognised: Paleoamerican, Mexican Plateau, Mountain Mesoamerican, Nearctic and Typical Neotropical. We undertook three cladistic biogeographic analyses on the basis of 49 cladograms of terrestrial taxa, partitioning them into three time-slices, namely, Miocene (Mountain Mesoamerican cenocron), Pliocene (Mountain Mesoamerican plus Nearctic cenocrons) and Pleistocene (Mountain Mesoamerican, Nearctic and Typical Neotropical cenocrons). For the Miocene time-slice, we observed a close relationship of the Transmexican Volcanic Belt with the Neotropical region, whereas, for the Pliocene and Pleistocene time-slices, the closest relationship of the Transmexican Volcanic Belt was with the Nearctic region. We conclude that the Transmexican Volcanic Belt may have played a different role according to the cenocron analysed, and that the Mexican transition zone differs in its delimitation depending on the taxa analysed, strengthening the idea that it is a complex area.